1. Field of Invention
The present invention relates to methods and apparatus for implementing diversity site group operations in air/ground communications and, more particularly, to methods and apparatus associated with diversity site group operations in an integrated digital voice/digital data communications environment.
2. Description of the Related Art
In air traffic control (ATC), air/ground (A/G) radio communication between a ground-base controller and an aircraft is typically supported by a ground station (GS) at a remote location. The controller is typically at a centralized control facility and manages the radio resources at the remote ground station via terrestrial telecommunication links. The aircraft in the domain of a ground station are referred to as a talk group. ATC A/G communication is based upon a simplex protocol in which the ground station transmitter and aircraft transmitters associated with a common talk group share a single A/G communication channel.
To extend the geographical area of A/G communication and/or to improve communication coverage within an identified geographical area controlled by a single controller, it is common to combine the airspace served by two or more radio communication ground stations so that a single controller can use the resources of multiple ground stations to manage a talk group. The multiple ground stations operating in this mode of operation are referred to as a diversity site group (DSG). FIG. 1 is a system-level diagram depicting a representative DSG 100. In a DSG, while all ground stations and aircraft in the talk group listen simultaneously for transmissions, only one ground station or aircraft transmitter can broadcast during any given period of time, because simultaneous broadcasts from multiple transmitters will typically interfere with each other and thus prohibit communications.
In the representative DSG 100 presented in FIG. 1, a control switch 102 is configured to communicate with multiple ground stations, wherein each ground station provides Very High Frequency (VHF) A/G communication to a geographical area of airspace. As shown in FIG. 1, a single control switch 102 communicates with geographic area 106 via ground station 104, communicates with geographic area 110 via ground station 108, and communicates with geographic area 114 via ground station 112. The challenge of managing a DSG is to have the control switch manage and coordinate the actions of the ground stations so that they appear to the aircraft in the talk group as a single ground station. By managing a DSG in this manner, airspace 106, airspace 110 and airspace 114 are effectively combined to form a single integrated geographic airspace 116 supported with A/G communication by the integrated ground stations of the DSG 100.
Very High Frequency (VHF) Digital Link Mode 3 (VDL-3) is a standard for integrated digital voice and data promulgated by the Radio Technical Commission for Aeronautics (RTCA) for A/G communications. The standard is based upon VHF carriers with 25 kHz spacing so as to be consistent with the current Double Side-Band Amplitude Modulation (DSB-AM) system for A/G analog voice. The symbol rate is 10.5 ksps and the symbol space is 8-Phase Shift Keying (8-PSK). Thus, each symbol carries 3 bits, and the supported bit rate is 31.5 kbps.
Each VDL-3 25 kHz RF carrier is composed of sequential time-division multiple access (TDMA) 240-msec Media Access Control (MAC) cycles. FIG. 2 is a schematic depicting a representative TDMA 240-msec MAC cycle 200. As indicated in FIG. 2, each 240-msec VDL-3 MAC cycle 200 is composed of two 120-msec frames (i.e., an even frame 202 and an odd frame 204). Each 120-msec frame is composed of four 30-msec time slots. In the representative MAC cycle shown in FIG. 2, even frame 202 contains time slot 206, time slot 208, time slot 210, and time slot 212. Odd frame 204 contains time slot 214, time slot 216, time slot 218, and time slot 220.
As indicated in FIG. 2, a representative 30-msec time slot (e.g., time slot-A of even frame 202) incorporates both a management (M) burst 222 and a voice or data (V/D) burst 224. The M bursts 222 are used by the ground station to provide a time reference, a channel status and for coordinating access to the M and V/D slots by aircraft. Aircraft use the M slots to register with the talk group and request channel resources. V/D bursts 224 are used by both ground stations and aircraft to transmit voice and data bursts.
In VDL-3, each 25 kHz RF carrier supports four separate TDMA channels, wherein each channel is defined by a time slot. Like the DSB-AM system it replaces, VDL-3 is a simplex communications standard whereby the ground stations and all the aircraft in a talk group communicate on a common channel (same frequency and time slot). Each of the four VDL-3 channels supported by a single 25 kHz RF carrier is supported by one of the four respective 30-msec time slots that make a 120-msec frame.
For example, referring again to FIG. 2, within a single MAC cycle, one of the four VDL-3 TDMA channels supported by a single 25 kHz RF carrier (e.g., channel-A) can be supported by time slot-A 206 within even frame 202 and by time slot-A 214 within odd frame 204. A second TDMA channel supported by the same 25 kHz RF carrier (e.g., channel-B) can be supported by time slot-B 208 within even frame 202 and by time slot-B 216 within odd frame 204. A third TDMA channel supported by the same 25 kHz RF carrier (e.g., channel-C) can be supported by time slot-C 210 within even frame 202 and by time slot-C 218 within odd frame 204. A fourth TDMA channel supported by the same 25 kHz RF carrier (e.g., channel-D) can be supported by time slot-D 212 within even frame 202 and by time slot-C 220 within odd frame 204.
DSG Management in Existing Analog DSB-AM Voice Systems
DSGs operations for existing AM voice systems are conducted primarily under the manual operation of a controller. As indicated in FIG. 1, only one ground station transmits at any given time, while all the ground station of a DSG receive simultaneously. Typically, the transmitting ground station is chosen by the controller from within the control site by configuration of the voice switch. While multiple ground station may relay a signal received from an aircraft to the control site, the controller chooses among the received signals. This choice can be made by the controller by manually configuring the controller switch, or may be determined automatically by the controller switch. Typically, a controller switch is configured by the controller to choose the first signal arriving at the switch that meets a defined signal quality.
DSG Management in Existing Digital Voice and Data Systems
VDL-3 is a mature specification of a digital voice and data system insofar as a single ground station is involved. However, at the current time, the specification does not address DSG operations. Additionally, there are no proposals that describe a complete solution to the problem. In VDL-3, avoiding simultaneous transmissions is more complicated than with DSB-AM because of a number of factors. For example, M slots and V/D slots are separate resources and are managed separately. Additionally, the M slots support signaling data transactions between radios that are orchestrated by computers, not the controller. Thus, it is clear that a method for managing the transmissions from the ground stations of a DSG is complex and involves both automated computer and controller actions. This is further underscored by the operations of VDL-3 with a single ground station. FIG. 3 is a system level block diagram depicting control of VDL Mode 3 transmissions using a single ground station configuration. FIG. 3 illustrates that a control site 302 manages the use of voice slots, but that the ground station 304 manages the utilization of management slots and data slots. Thus, while the voice slots are under the control of a human operator, a computer autonomously controls transmissions into the M and D slots. As previously described, the challenge of managing a DSG in VDL-3 is to have the control site manage and coordinate the actions of the ground stations so that the ground stations appear to the aircraft in the talk group as a single ground station.
Shortcomings of Existing Analog DSB-AM Voice Systems
One shortcoming of existing analog DSB-AM voice systems is that the controller is often presented with insufficient information by which to intelligently choose which ground station to transmit from and which ground station to receive from. FIG. 4 is a system level diagram depicting selection of a ground station signal by an analog controller switch. FIG. 4 illustrates a case where a weak radio signal, received from an aircraft 402 at a first ground station 404 of a DSG, arrives at a voice switch 408 of a control site before a stronger radio signal received from the same aircraft 402 at a second ground station 410 of the DSG. In this representative example, the stronger signal is delayed in reaching voice switch 408 due to a longer telecommunication delay between the control site and the second ground station 410. Such a condition can result in using a poor communication path to receive an incoming signal when a good communication path is available. Often, a controller will choose to transmit an outbound signal over the same ground station chosen to receive an incoming signal. Thus, if a poor choice is made for the receive ground station, the same poor choice will likely be made for the transmit ground station.
Shortcomings of Existing Digital Voice and Data Systems
Existing digital voice and data systems have a shortcoming similar to that described above. Information available to the controller by which to intelligently choose which ground station to receive from and which ground station to transmit from is often unavailable. The issue with respect to existing digital voice and data systems is essentially the same as that described above for DSB-AM voice systems. However, the consequences of a poor choice of transmitter can be more severe in digital voice and data systems. For example, in the VDL Mode 3 system, an aircraft does not accept M or V/D bursts that arrive ±1 symbol offset from perfect timing. Thus, if an aircraft receives timing from a first ground station (i.e., GS1), it will not accept transmissions from a second ground station (i.e., GS2) if the time-of-flight path difference from the aircraft to the two ground station is greater than 1 VDL-3 symbol. At 10.5 ksps, this corresponds to path difference of 17.86 miles in distance.
A second shortcoming with respect to existing digital voice and data systems is a lack of a method to coordinate ground station transmissions in M slots. A beacon is an uplink M burst that occurs in the middle of the VDL-3 240 msec Media Access Control (MAC) cycle. A single ground station (not operating within a DSG) transmits a periodic beacon M burst as indicated in FIG. 5. FIG. 5 depicts representative M and V bursts in a VDL Mode 3 TDMA frame (voice only) configuration. Within a DSG, the ground stations need coordination so that only one ground station transmits at the same time. The beacon provides system time to all aircraft in the talk group. In VDL-3 it is referred to as Logical Burst Access Channel (LBAC) 5. VDL-3 has a corresponding downlink management slot that aircraft transmit on that is called LBAC 1. FIG. 5 illustrates LBACs 1 and 5 in the context of a voice only service. VDL-3 voice and data service configurations have a richer set of LBACs. A single ground station (not operating within a DSG) that supports voice and data service transmits a periodic beacon M and V bursts as indicated in FIG. 6. FIG. 6 depicts representative M and V bursts in a VDL Mode 3 TDMA frame (voice and data) configuration. In this configuration, slots A and C are the voice and data channels for a single talk group, respectively. LBAC 5 is still the only ground station Management burst, but aircraft have access to LBACs 3 and 7 as well as LBAC 1.
A third shortcoming associated with existing digital voice and data systems is a lack of a method to coordinate ground station assignment of local addresses. In VDL-3 an isolated ground station has the autonomy to assign a temporary local ID to each aircraft while the aircraft is in its domain. However, within a DSG, all ground stations share a single address space of local IDs. Thus, a system for coordinating and sharing the address space by ground stations of a DSG is needed.
Additional shortfalls associated with existing digital voice and data systems include: lack of a method for tracking the status of communication channels between aircraft and ground stations of a DSG; lack of a method for controlling A/G communication handovers between the ground stations of a DSG; and lack of a method for routing addressed messages within a DSG. With respect to the routing of addressed messages, no method is specified in VDL-3 for determining through which ground station of a DSG an aircraft can be reached. Such a capability is necessary for assigning polling responsibility to a ground station as well as routing messages addressed to an aircraft to the correct ground station. As a corollary, no method is described in VDL-3 for handing over polling responsibilities of an aircraft among the ground station in a DSG.
Yet another shortfall associated with existing digital voice and data systems is a lack of a method to coordinate ground station transmissions in data slots. In VDL-3, a ground station that is not part of a DSG has the autonomy to manage and assign data slots to aircraft that make reservation requests on downlink M slots. However, within a DSG, all ground stations share a single set of data slots. Therefore, a system for coordination and sharing of the data slots by ground stations of a DSG is needed.
Accordingly, there is a need for methods and apparatus that allow a digital voice and data A/G communication system controller to: intelligently choose which ground station to transmit from and which ground station to receive from; coordinate M slot transmissions and data slot transmissions by ground stations within a DSG; coordinate the assignment of local addresses assigned to aircraft by ground stations within a DSG; track the status of A/G communications between an aircraft and the ground stations of a DSG; control A/G communication handovers between the ground stations of a DSG; route addressed messages along a desired path within a DSG; and, control the handover of polling responsibilities of an aircraft between ground stations in a DSG.